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Phases of the Moon



What you see when you look at the moon depends on its location in relationship to the sun and Earth. The moon never goes away or changes shape-we just see a different fraction of sunlight being reflected from the moon to Earth. So how do you explain why this happens? Start with the facts: The moon is our planet's only natural satellite. Its diameter is about a quarter that of Earth's. The moon takes about 27.3 days (about a month) to revolve around Earth, traveling at an average distance of about 384,000 kilometers. We divide the moon's orbital cycle into several segments, or phases. When the sun and the moon are on the same side of Earth, the sun illuminates the side of the moon that faces away from Earth. We don't see any reflected sunlight on its front face, so it looks like there is no moon. We call this the new moon phase. When the crescent moon begins to appear, if you look carefully you may see some faint illumination of the moon from earthshine. About two weeks later, when the moon and sun are on opposite sides of Earth and all are in a line, the sun shines past Earth directly onto the full face of the moon and we see a "full moon." What happens in between? As the new moon phase ends, the moon waxes, or appears to grow larger, and we see more of the moon's face. The lighted area increases over time from right to left from our perspective on Earth. When the sun­earth­moon angle is very small, we see only a thin bright curve, called the waxing crescent. Over the next seven days the angle between the sun, Earth, and the moon grows to 90 degrees. We see the sunlight spread to cover the right half of the moon. This is called the first quarter. The visible part of the moon continues to wax through the gibbous phase over the next seven days until we see the full moon. As the cycle continues, we say the moon is waning, or growing smaller. The amount of lighted area we see decreases, and the darkened area increases from right to left. You can tell if the moon is waxing or waning by whether the right side of the moon is dark or light. Another 14 days pass as the moon moves through the waning gibbous phase, then the third quarter, then the waning crescent phase, and seems to finally disappear in the new moon phase. Now we're back to where we started about a month ago!


Work in teams to create 2D and 3D models of each phase of the moon. After creating, testing, and refining your models, you'll be able to demonstrate to your classmates what you've learned. Materials
  • one 5­ to 10­cm Styrofoam ball (the moon)
  • a light source (the sun), such as an overhead projector or lamp with a 400­watt bulb
  • room that can be darkened
For each group of students, provide:
  • chart paper
  • markers
  • rulers
  • one 2­cm Styrofoam ball
  • one 4­cm Styrofoam ball
  • toothpicks
  • large flat sheet of foam core or Styrofoam packing material
  • flashlight
1. As an introduction to this activity, view the NEWTON'S APPLE video about the phases of the moon. Emphasize and review the direction of Earth's rotation and the moon's revolution. 2. Work in groups of three or four. Assign a phase of the moon to each group:
A. NewB. Waxing CrescentC. First QuarterD. Waxing Gibbous E. FullF. Waning GibbousG. Third QuarterH. Waning Crescent
3. Use markers to draw a diagram on a piece of paper that shows the position of the moon, sun, and Earth during your assigned phase of the moon. Be sure to label the diagram to indicate the names of each of the bodies as well as the name of the phase. 4. Create a 3D model of your diagram. Use toothpicks to attach the Styrofoam earth and moon balls to the flat sheet of foam core. 5. Use a flashlight to provide the sunlight. Darken the room when everyone is ready to test their model. Move the balls as necessary to get the correct phase. Mark and label the positions of the flashlight, moon, and Earth on the foam base when the correct phase is attained. 6. Now for the real test: Explain to the class why we see your phase of the moon. Use your diagram and 3D model. Darken the room and role­play the parts of sun (overhead projector/light bulb), Earth (volunteer from class), and moon (the large Styrofoam ball). Do not state which phase you are demonstrating. Ask a volunteer to guess, based on what he or she sees on the "moon." Questions 1. What do you think it would be like to have several moons revolving around Earth? Would it change your calendar? Poetry? Tides? 2. What views do you think astronauts have of Earth and moon as they orbit Earth? 3. Would the moon phases change if the moon revolved around Earth in the opposite direction? How?